Environmental Engineering Reference
In-Depth Information
retest in the same ear and at the same frequency. NIOSH recommends a
confirmation audiogram within 30 days, with the confirmation audiogram
being preceded by a quiet period of at least 14 hours.
Threshold shift
—Audiometric monitoring programs will encounter two types of
changes in hearing sensitivity (i.e., threshold shifts): permanent threshold shift
(PTS) and temporary threshold shift (TTS). As the names imply, any change
in hearing sensitivity that is persistent is considered a PTS. Persistence may
be assumed if the change is observed on a 30-day follow-up exam. Exposure
to loud noise may cause a temporary worsening in hearing sensitivity (i.e., a
TTS) that may persist for 14 hours (or even longer in cases where the expo-
sure duration exceeded 12 to 16 hours). Hearing health professionals need to
recognize that not all threshold shifts represent decreased sensitivity, and not
all temporary or permanent threshold shifts are due to noise exposure. When
a permanent threshold shift can be attributable to noise exposure, it may be
referred to as a
noise-induced permanent threshold shift
(NIPTS).
Velocity
—The speed at which the regions of sound producing pressure changes
move away from the sound source.
Wavelength (l)
—The distance required for one complete pressure cycle to be
completed (1 wavelength), measured in feet or meters.
Weighted measurements
—Two weighting curves are commonly applied to mea-
sures of sound levels to account for the way the ear perceives the “loudness”
of sounds.
A-weighting
—A measurement scale that approximates the “loudness” of
tones relative to a 40-db SPL, 1000-Hz reference tone. A-weighting has
the added advantage of being correlated with annoyance measures and
is most responsive to the mid-frequencies, 500 to 4000 Hz.
C-weighting
—A measurement scale that approximates the “loudness” of
tones relative to a 90-dB SPL, 1000-Hz reference tone. C-weighting has
the added advantage of providing a relatively flat measurement scale
that includes very low frequencies.
Noise Exposure
Noise literally surrounds us every day and is with us just about everywhere we go;
however, the noise we are concerned with here is that produced by wind turbines.
Excessive amounts of noise in the wind farm environment (and outside of it) cause
many problems for people, including increased stress levels, interference with com-
munication, disrupted concentration, and, most importantly, varying degrees of
hearing loss. Exposure to high noise levels also adversely affects quality of life and
increases accident rates.
DID YOU KNOW?
Windmills have been in use since 200 BCE and were first developed in Persia
and China. Ancient mariners sailed to distant lands by making use of winds.
Farmers used wind power to pump water and for grinding grains.
